(1) Field of the Invention
This invention relates to woven fabrics and especially to paper forming fabrics.
(2) Description of the Prior Art
In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers, known as the paper "stock" is fed onto the top of the upper run of a traveling endless belt of woven wire and/or synthetic material. The belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium to form a wet paper web. In forming the paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for the drainage of the aqueous medium through the mesh openings of the belt, known as drainage holes, by vacuum means, or the like, located on the machine side of the belt, or "fabric". After leaving the forming section, the paper web is transferred to a press section of the machine, where it is passed through a series of pressure nips formed by cooperating press rolls to remove still more of the moisture content. The paper is then transferred to a dryer section for further moisture removal.
Such papermakers' fabrics are manufactured in accordance with two basic methods to form an endless belt. They are flat woven by a flat weaving process with their ends joined by any one of a number of well known methods to form an endless belt. Alternatively, they are woven directly in the form of a continuous belt by means of an endless weaving process. In a flat woven papermakers' fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In a papermakers' fabric having been woven in an endless fashion, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein the terms "machine direction" and "cross machine direction" refer, respectively, to a direction equivalent to the direction of travel of the papermakers' fabric on the papermaking machine, and a direction traverse to the direction of travel. Both methods are well known in the art and the term "endless belt" as used herein refers to belts made by either method.
Effective sheet support and lack of wire marking are important considerations in papermaking, especially for the forming section of the papermaking machine where the wet web is formed. The problem of wire marking is particularly acute in the formation of fine paper grades where the smoothness of the sheet side surface of the forming fabric is critical. Marking affects a host of paper properties, such as sheet mark, porosity, see through, pin holing, and the like. Accordingly, paper grades intended for use in carbonizing, cigarettes, electrical condensers, quality printing, and like grades of fine paper, have heretofore been formed on very fine woven forming fabrics or fine wire mesh forming fabrics. In order to ensure good paper quality, the side of the papermakers' fabric which contacts the paper stock must provide high support for the stock, preferably in the cross machine direction, because paper fibers delivered from a headbox to the forming fabric are generally aligned in the machine direction more so than in the cross machine direction. Trapping these paper fibers on the top of the forming fabric during the drainage process is more effectively accomplished by providing a permeable structure with a co-planar surface which allows paper fibers to bridge the support grid of the fabric, rather than align with the support grid. By "co-planar" is meant that the upper extremities of all yarns defining the paper forming surface are at the same level, such that at that level there is presented a substantially "planar" surface.
Such forming fabrics, however, may often be delicate and lack stability in the machine and cross machine directions, leading to a short service life. Abrasive and adhesive wear caused by contact with the papermaking machine equipment constitutes a substantial problem. The side of the papermakers' fabric which contacts the paper machine equipment must be tough and durable. Such qualities, however, most often are not compatible with the good drainage and fiber supporting characteristics desired for the sheet side of a papermakers' fabric.
In order to meet both standards, two layers of fabric can be woven at once by utilizing threads of different size and/or count per inch and another thread to bind them together. This fabric is commonly called a double layer fabric. Alternatively, fabrics have been created using multiple layers to insure that the fabric has desirable papermaking qualities on the surface that faces the paper web and desirable wear resistance properties on the machine contacting surface. For example, papermakers' fabrics may be produced from two separate fabrics, one having the qualities desired for the paper contacting side and the other with the qualities desired for the machine contacting side, joined together by a third set of threads. This type fabric is commonly called a triple-layer fabric. Generally, these structures do not possess the high level of stretch resistance desired in a papermaking fabric. Furthermore, the yarn that binds the fabric together will often produce a sheet mark, often from the long machine direction floats. Accordingly, no known fabrics have achieved the qualities necessary to meet the competing standards to produce superior paper.
In U.S. Pat. No. 4,987,929, issued Jan. 29, 1991, in the name of Robert G. Wilson, there is provided an improved papermakers' fabric for use in a papermaking machine, including an initial fabric layer having single float machine direction knuckles on the paper contacting surface and into which are woven additional fiber supporting cross machine direction yarns, preferably of smaller diameter than the fabric layer yarns. The additional fiber supporting cross machine direction yarns are held in place centrally between adjacent fabric layer cross machine direction yarns by additional cross machine direction locator yarns, generally being of approximately the same diameter as the fiber supporting yarns. The papermakers' fabric of the '929 patent may be a single-layer, double-layer or triple-layer fabric.
The forming fabric shown and described in the '929 patent has proven effective, but is limited to floats of odd numbers in the arrangement of the additional yarns. Odd numbered floats are acceptable and in fact necessary when the fabric is formed on an eight harness loom. However, if the fabric is formed on a ten harness loom, it is usually the case that floats of even numbers are utilized, for example, floats extending over four yarns, under a single yarn, over another four yarns, etc.
Accordingly, there is a need for a paper forming fabric which provides the benefits of the fabric shown and described in the '929 patent, but which, in addition, may be made on a ten harness loom and may, therefore, include even numbered floats for the additional yarns.